Binding Energy
Binding energy is the term used to specify the amount of energy necessary to hold components of a system together, usually in describing the interactions within constituents of molecular, atomic or nuclear systems. However, it can also be used in describing astrophysical systems. Through binding energy, the system is relatively stable and allowed to act as a unit. The measure of the energy necessary to hold a system together is essentially equal to amount of energy necessary to break the system apart. Therefore "binding energy" is used in reference to both processes.
At the astrophysical level, the force of gravity provides much of the binding energy necessary to keep solar systems as units. Gravity holds the components of a system together because no matter how small the system's "particle" (Newton's term) is, it will have an attraction to another object proportional to the mass of the two objects. This attraction of the two objects is inversely limited by the distance between the two objects. As massive as Earth and the Sun are, they will exert a strong attraction on each other. Because Earth and the other planets rotate around the Sun, there are forces such as angular momentum and centrifugal force that prevent the planets from leaving their orbits and crashing into the Sun.
Binding energy among atomic particles prevents the atom from breaking down into a number of hydrogen atoms with extra neutrons. Francis Aston (1877-1945) investigated the phenomenon of atoms weighing less than the sum of their particles. He calculated the total weight of the hydrogen atoms and neutrons that would be produced by the atom breaking apart minus the weight of the original atom. His relating the mass of the constituent parts of an atom to the mass of the whole atom identified a slight difference. Aston termed this difference the packing fraction. He concluded that in bringing the constituent particles together to form the atom some matter was transformed into energy. An equivalent amount of energy would be necessary to split the atom apart. Albert Einstein calculated this energy as equaling the mass defect (the difference between the sum of the mass of the atomic particles and the mass of the atom as a whole) times a constant equaling the square of the speed of light.
At the particle level binding energy is measured as the difference of the mass of the whole nucleus and the sum of the constituent parts. When the binding energy of the nucleus is determined, oftentimes this difference is divided by the number of nucleons for a convenient average per nucleon. A nucleus with low binding energy and low mass may reconfigure with other structures of low stability to form a more massive and stable structure through the process of nuclear fusion. A nucleus with high mass and low stability may intercept a neutron and begin a process known as fission. During fission, a nucleus splits into two nuclei of less mass with greater stability. The more binding energy the structure has, the less likely it is to reconfigure or breakdown into another structure. Binding energy is increasingly low for atoms with an atomic mass greater than 100. Fission is more likely to occur in these atoms since all matter tends to progress toward a more stable state of high binding energy. Among the elements, the standard of stability is set by iron and nickel with atomic masses of 50 and 60, respectively. Elements with atomic mass less than that of iron or nickel have extremely low binding energy. Among these elements, fusion is more likely to occur.
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